The present invention relates to improvements in apparatus for intermittently operating pivotable or otherwise movable driven members by a rotary cams. More particularly, the invention relates to improvements in apparatus which can be utilized in packing machines, such as cigarette packing machines, for intermittently actuating heated sealing tools which weld sheets or parts of sheets of heat-sealable (particularly synthetic thermoplastic) material to each other.
In many types of cigarette packing machines, successive packs which contain arrays of cigarettes are delivered into the range of one or more heated sealing tools which are actuated to engage the wrappers of successive packs and to thereby bond flaps, tucks or otherwise configurated parts of wrappers to one another. For example, a heated sealing jaw can be pivoted into sealing engagement with portions of polypropylene wrappers for successive cigarette packs which are transported by an indexible turret. The movements of the sealing jaw into engagement with successive wrappers are synchronized with indexing movements of the turret to thus ensure that the jaw engages a fresh wrapper whenever the turret is brought to a standstill. Such machines are further provided with apparatus which ensure that the sealing jaw is held in heat-transmitting engagement with successive wrappers for identical intervals of time irrespective of the (variable) speed of the machine, i.e., independently of the frequency at which the turret is indexed with reference to the jaw. Since the means for pivoting the sealing jaw into and from engagement with successive wrappers receives motion from the prime mover which indexes the turret, such pivoting means must be adjusted whenever the speed of the prime mover is changed.
An apparatus which can ensure that the periods of dwell of the sealing tool in heat-transmitting engagement with successive wrappers will remain unaffected by the speed of the prime mover for (a) the means which moves the tool and (b) the turret or another suitable conveyor for the wrappers is disclosed in German Offenlegungsschrift No. 19 65 693. A rotary disc cam is tracked by a roller follower on a first lever which transmits motion to a two-armed second lever through the medium of a link. One arm of the second lever is adjustably connected to the respective end portion of the link, and the other arm of the second lever carries the heating tool. The apparatus of this German publication further comprises a device which can shift the one arm of the second lever lengthwise of the link against the opposition of a coil spring to thereby change the inclination of the second lever relative to the first lever in dependency on changes in the speed of the cam. Such shifting of the one arm of the second lever entails a movement of the sealing tool away from its operative position if the second lever is thereby pivoted from an operative position in which the sealing or heating tool is ready to engage a workpiece. A timer which is operated in synchronism with the moving parts of the machine is used to actuate a valve which is installed in a conduit serving to admit fluid to a cylinder whose piston rod constitutes a means for shifting the one arm of the second lever against the opposition of the coil spring so that the period of heat-transmitting engagement between the heating tool and a workpiece is determined by the timer, i.e., by the momentary machine speed which is proportional to the RPM of the disc cam. The machine which embodies the just described apparatus is designed to produce plastic bags and similar receptacles.
The preamble of the aforementioned German publication further refers to heating or sealing tools which are actuated exclusively by pneumatic means.
German Offenlegungsschrift No. 15 11 626 discloses heating or sealing tools which are actuated by timer-controlled pneumatic means or by electomagnets.
The just discussed conventional apparatus are not suitable for use in many recent types of packing machines, such as modern cigarette packing machines which are designed to turn out up to 500 packs per minute. This is due to the fact that a timer-controlled pneumatic actuator or an electromagnetic actuator is either too slow or too weak to reliably actuate the sealing tool or tools at 500 cycles per minute. Moreover, the useful life of such conventional apparatus is relatively short which renders them impractical for use in a modern high-speed cigarette packing machine because even a short-lasting interruption of operation of such a machine for the purpose of inspecting, repairing or replacing the actuating apparatus for one or more sealing tools would entail inordinately large losses in output. This will be readily appreciated by bearing in mind that an average cigarette pack contains twenty cigarettes.